The opossum of the genus Didelphis is one of the principal wild reservoirs of Trypanosoma cruzi and is widely distributed in the Western Hemisphere. Didelphis albiventris is the most common marsupial in Amamá and Trinidad, two communities in the province of Santiago del Estero, Argentina. The D. albiventris population is replaced every year, and the opossum normally has two reproductive periods, one at the beginning of the spring and another at the beginning of the summer. The two litters are weaned, and they leave the mother's marsupial pouch to join the population, the first (G1) at the beginning of the summer and the second (G2) at the beginning of the fall. Between 1988 and 1991 409 D. albiventris opossums were studied, and xenodiagnoses showed that 35% of them were infected with T. cruzi. Annual cycles of renewed infection were observed, with prevalences that ranged between 22% and 43%. The acquisition of the parasite occurred over the entire year, from the summer through the spring. The prevalence of infection increased with age. The G1 individuals tended to present higher prevalences than the G2 individuals, probably from being exposed to transmission for a longer period of time. In the first two (younger) age categories for the opossums, G2 individuals showed higher prevalences than did the G1 individuals. This indicates a significant increase in transmission intensity during the fall. Opossums should be regarded as a potential source of T. cruzi entry to the domestic transmission cycle.